1. Field of the Invention
This invention relates generally to a horizontal-flow hydration apparatus and a method of using such equipment to make hydrated polymers and viscous aqueous fluids containing such hydrated polymers. Such viscous fluids are widely used in the oilfield service industry as a primary component in fracturing fluids, acidizing fluids and gravel packing fluids. The invention is particularly useful in making hydrated viscous fluids in a continuous-mix process for fracturing fluids.
2. Description of Related Art
The oilfield service industry uses viscous fluids extensively in treating subterranean earth formations to stimulate the production of oil and gas (i.e., natural gas) from the formation. Examples of such treatment include fracturing, acidizing and enhanced oil recovery. The industry also uses such viscosified fluids in operations to complete the well, such as gravel packing.
The polymers used to prepare such fluids are well known. The class of polymers has many members, all of which are hydratable. The most widely used polymers are guar (aka guar gum) and derivatives of guar, such as hydroxypropylguar (“HPG”), carboxymethylhydroxypropylguar (“CMHPG”), and the like. Guar is a long-chain, high molecular weight polymer composed of mannose and galactose sugars, and is therefore a polysaccharide. Other polymers include, by way of example and without limitation, polyacrylamide, acrylamide copolymers and derivatives of such polyamides.
Water-based fracturing fluids thickened with guar have been used since the late 1950s. It is estimated that today more than two-thirds of the fracturing treatments that use water-base gels are viscosified with guar or HPG. The polymers are typically sold as dry powders (i.e., as dry particulate solids) in containers of convenient size. Bags of polymer weighing 50 pounds or “supersacks” weighing 2 to 3 thousand pounds are commonly used. The polymers have also been supplied as a slurry in a non-aqueous medium. As used herein, the term “slurry of a hydratable polymer” shall mean a pumpable slurry comprised of a hydratable polymer dispersed in a hydrophobic fluid, such as diesel.
Historically, the viscous fluids have been prepared on site in batch-mix operations or in continuous-mix operations. Previous commercial mixing procedures encountered various problems. For example, even the logistics of providing large volumes of fluid to well sites, which for some reason generally seem to be at some “remote” location, in consistent quality and quantity are daunting. Mixing dry additives (including hydratable polymers) with water or other aqueous fluids is problematic and special equipment and operating procedures are generally required to avoid chemical dusting, uneven mixing, lumping or gels. Extended preparation and mixing time are involved. Furthermore, with respect to batch-mix operations, job delays can result in the deterioration of pre-mix gels of the hydratable polymers, especially guar and guar derivatives, and the potential loss of the time and materials incurred in preparing the gels. Batch operations also suffer chemical losses due to tank bottoms and costs associated with pre-treatment and post-treatment tank clean-up. For these and other reasons, the industry has evolved to a preference to continuous-mix operations, especially for fracturing treatments. The slurries of hydratable polymers mentioned above are especially well adapted for use in continuous-mix operations.
U.S. Pat. No. 4,828,034, entitled “Method of Hydrating Oil Based Fracturing Concentrate and Continuous Process Using Same,” by Vernon G. Constein and Harold D. Brannon, described one method of mixing a fracturing fluid slurry concentrate comprising a dispersed hydratable polymer (e.g., HPG) and a hydrophobic solvent base (e.g., No. 2 diesel) on a real time basis during the fracturing treatment of a subterranean formation. The process was said to produce a fully hydrated fracturing fluid in a continuous operation without the use of large volume storage/hydration tanks.
U.S. Pat. No. 5,046,856, entitled “Apparatus and Method for Mixing Fluids,” by William R. McIntire, described a novel apparatus and method that advanced the technology over U.S. Pat. No. 4,828,034. In the '856 patent, a slurry concentrate was dispersed in water and the resulting fluid passed through a series of interconnected vertical tanks, at least one of which sheared the polymer slurry and water feed with a radial flow impeller positioned parallel to an axial fluid flow path. The process was said to provide a “plug flow” of materials through the plurality of vertical tanks and to effect complete hydration of the hydratable gel for use in well treatment operations. This process was subsequently referred to by McIntire and the patent assignee, Dowell Schlumberger Inc., as the “Precision Continuous Mix Process” or “PCM Process.”
U.S. Pat. No. 7,223,013, entitled “First in First out Hydration Tanks,” by Thomas E. Allen, described a design for a hydration tank that allegedly reduced the amount of stagnation that can occur in hydration tanks. This can be a problem when attempting to hydrate polymers such as guar and HPG, even in the PCM Process of McIntire. The partially hydrated polymers are not Newtonian fluids and they tend to form “channels” while passing through the hydration tanks. The less viscous fluids entering the tank tend to bypass the more viscous fluids through channels of least resistance. As Allen states, if the fluid is not managed properly, parts of the tank will become gelled and motionless and will be difficult to get moving again. When gelation occurs, the gelled fluid remains in one place and the newly mixed fluids that enter the tank will bypass the gelled fluid. Thus, the tank is functionally smaller than its actual size since part of the fluid in the tank is not moving.
J. E. Brown et al., Schlumberger Dowell, published a paper entitled “Fracturing Operations” in the text “Reservoir Stimulation,” Third Edition (2000), edited by Michael J. Economides and Kenneth G. Nolte, John Wiley & Sons, Ltd., page 11-23, which stated: Process-controlled blending equipment that meters and continually mixes polymer slurry, concentrated potassium chloride solution and liquid additives has made continuous-mix operations a viable alternative to batch-mix operations. There are several advantages to performing a fracture treatment in continuous-mix mode. Environmental concerns are greatly reduced because only freshwater residuals remain in the fracture tanks after a treatment. Besides eliminating the cost of gelled tank bottom, no tank cleaning or disposal costs are incurred. In addition, a more predictable and consistent viscosity is obtainable for large treatments, where bacteria can degrade the gel viscosity of a batch-mixed fluid before pumping begins. Personnel time and costs can also be greatly reduced. The continuous-mix process eliminates the need to have gelling crews precede fracturing operations, resulting in direct savings in time for personnel and equipment. Finally, viscosities can be easily changed throughout the treatment. This allows tapering the polymer loading so that fluid damage to proppant conductivities can be minimized or a net pressure limitation can be met.
To ensure that a continuous-mix operation goes smoothly, several requirements must be observed. The polymers should be metered accurately either as dry powders or powders slurried in a carrier fluid. The polymers should be thoroughly dispersed in the water either by educting the dry polymer powder into the water stream or injecting the slurried polymer powder into the water stream and providing agitation to disperse the slurry in the water. Specialized mixing and hydration units should provide the metering capabilities, proper shear environment a sufficient residence time for proper hydration. The hydration process related time and shear has proved to be extremely important for continuous-mix treatments. If the base fluid has not progressed sufficiently in the hydration process before the liquid is crosslinked, the fluid will have decreased viscosity and may experience stability problems.
The hydration process requires time. Batch mixing tests of the hydration process show that adding high-shear mixing accelerates the process of hydration. The process is also temperature dependent, with lower fluid temperatures slowing the process. The time required for hydration of powdered guar, either dispersed in a carrier fluid to be injected into the aqueous stream or dispersed directly as a dry powder into the aqueous stream through an eductor or other means, is typically on the order of a few minutes. Fracturing treatments for oil and gas wells require high pump rates, typically 20 to 100 barrels per minute. Large volume hydration tanks are required to provide adequate residence time for hydration at those rates.
Fracturing operations involve mobile equipment taken from wellsite to wellsite. Such equipment is normally transported on the public roadways, and thus requires equipment sizes and configurations in compliance with Federal and State Departments of Transportation rules and regulations. Size and weight restrictions place critical limitations on equipment design. Space for hydration tanks on trailers meeting such standards is limited, and it is advantageous to provide hydration equipment that makes optimum use of the space available.
It is common, but incorrect, to assume that the residence time for fluid flowing through a tank with volume, V, is given by that volume divided by the fluid flow rate, Q. That calculation is correct for the average residence time, but is not correct for each incremental volume of fluid, unless the fluid flow through the tank is “first-in-first-out” (FIFO), also referred to as “plug-flow”. While near FIFO conditions can be achieved under conditions of turbulent flow in a pipe, that condition does not happen in large volume tanks. Even under conditions of turbulent flow, eddy currents develop in large tanks; such eddy currents partially stagnate portions of the tank volume and lead to a spread in residence times (or “ages”) of different incremental fluid volumes exiting the tank. The spread in the residence times can cause the various incremental volumes of fluid to have quite different properties, especially for the viscous fluids thickened with guar or guar-based materials.
Fluids used for oil and gas well fracturing treatments (“fracturing fluids”) made from polymers suspended in water, have distinct rheological characteristics which make achieving FIFO conditions in a large tank essentially impossible. These fluids suppress turbulence through interactions of the suspended long-chain polymers. When the polymers are hydrated and turbulence is suppressed, flow in the tanks is in the laminar regime, developing velocity gradients and stable flow streams. These fracturing fluids are shear thinning; their viscosities decrease with shear. If a portion of the fluid in a tank is moving slowly, bypassed by the main stream of fluid, the slow moving fluid in its low shear environment has higher viscosity. This tends to slow the fluid further and stabilize stagnated regions of the tank. Eddy formation also results in slower moving fluid which has higher viscosity and tends to stabilize the eddy.
Different residence times for fluids is a problem particularly at start-up times. Young, partially hydrated fluid entering a tank of older, more hydrated fluid will have a lower viscosity and will tend to stay segregated from the more viscous fluid. The newly injected fluid channels through the more viscous fluids in the tank and exits after much less than the average residence time. This is particularly problematic when the tank contains fully hydrated fluid before a fracturing treatment begins or when the fracturing treatment has been stopped for a period of time and restarted. Fracturing treatments can be interrupted for a variety of operational causes or by the operator wishing to change the treatment procedure.
The hydration of polymer is not a linear function of time, but starts rapidly and proceeds more and more slowly as it approaches full hydration. This results in lower average viscosity of the discharge fluid than one would calculate using the hydration kinetics for a batch process and the average residence time in the tank. The loss in viscosity for a volume of fluid exiting the tank at less than the average residence time is greater than the increase in viscosity for a similar volume of fluid exiting after a longer than average residence time. The low viscosity of young exiting fluid is not compensated by the slightly higher viscosity of old exiting fluid.
To produce well-hydrated fluid on mobile equipment at the high pump rates required for fracturing treatments, it is advantageous to provide the high-shear conditions required to accelerate the hydration process and to control the flow of hydrating fluid through a large tank volume so that all volumes of fluid passing through the tank have sufficient residence times in the tank to fully hydrate. Without adequate control of the fluid flow, larger tank volumes are required to ensure adequate residence time for all the fluid, and even this may not be adequate if portions of the fluid are stagnant.
The hydration tanks described in U.S. Pat. No. 4,828,034 and U.S. Pat. No. 5,046,856 are vertical-flow compartmentalized tanks with generally vertical flow paths over and under walls separating tank compartments have several disadvantages compared to a horizontal-flow tank. Flow into the compartmentalized tank comes initially from a centrifugal pump. After that, flow from compartment to compartment is by gravity. Half the passages are over weirs between compartments and the other half through passages near the tank bottom. When discharge rates increase, the last (discharge) compartment level will drop until sufficient fluid head differences exist across all passages to force fluid to flow through the tank at the increased rate. The resulting delay in response to rate changes makes level control difficult.
On fracturing treatments it is essential that fluid flow be maintained to equipment downstream of the hydration unit at all times. When there is an interruption to the input flow to a vertical-flow compartmentalized tank, the tank level drops. Only the volume in the last compartment is available to be discharged to the equipment downstream. In a horizontal-flow compartmentalized tank, the entire volume of the tank is available for discharge in the case of inlet supply interruption. In the case of a brief inlet supply interruption, it is difficult to “catch up” by increasing the inlet rate to a vertical-flow compartmentalized tank. Each compartment must be filled to a level at which it flows to the next compartment at a rate required to exceed the discharge rate and refill the tank. Fluid flow to the last (discharge) compartment does not increase until fluid volumes in each of the upstream compartments are increased sufficiently to raise the flow rate through the tank. At high flow rates, significant drops in fluid level occur between compartments from the first compartment to the last. This reduces the effective volume of the tank for high rates at which lower residence time exists, and reduces that residence time further. Lastly, there is no natural flow path between pairs of compartments in such vertical-flow tanks. To empty the hydration tank at the end of a job requires valves and/or piping between pairs of compartments. This adds mechanical and operational complexity.
If hydration performance were not an issue, field operations personnel would prefer a horizontal-flow compartmentalized tank over a vertical-flow compartmentalized tank.
Some horizontal-flow compartmentalized hydration units have been built with long horizontal channels separated by vertical walls from the front of the tank to the rear. When water flows through those channels at fracturing operation rates, Reynolds numbers are high, and it appears that the flow of water down the channels could provide a reasonably FIFO flow stream. However, when pumping viscosified fluids with the turbulence suppressing characteristics of typical fracturing fluid polymers, laminar flow profiles develop in the channels. This leads to several problems: Large eddies are formed, thick fluid stagnates in portions of the tank, and less-hydrated fluid from the inlet channels through the tank to the discharge. This channeling reduces the degree of hydration and the viscosity of the discharge fluid. The lack of high shear mixing to accelerate hydration kinetics results in poor hydration unit performance of these horizontal-flow compartmentalized hydration units, especially at polymer concentrations normally required in hydraulic fracturing fluids.
Even though there have been many modifications in equipment and procedures to hydrate polymers for use as viscosified aqueous fluids in oilfield applications, a need still exists for an apparatus and method that can hydrate the polymers rapidly and consistently in real time. This is one of the objects of this invention.
Another object of this invention is to provide a horizontal-flow hydration tank that is less complex mechanically and operationally than the compartmentalized vertical-flow tanks. It is also anticipated that the present invention will permit the user to reduce the variable costs associated with making and using a viscosified fluid in fracturing treatments and other oilfield procedures to treat subterranean formations. That is, variable costs such as mixing and blending charges, transportation, storage and disposal charges, and power requirements should be reduced.